Your character doesn't fit with the guild
The obvious example of this is the upright and lawful paladin trying to join a guild of thieves, but the contrast is rarely that severe. More often it's a case in which you clearly want to take your character in a certain direction, but the rest of the guild wants to be doing something entirely different.
By way of example, let's say you're playing a Light-side Sith Inquisitor in Star Wars: The Old Republic, a character predicated on the idea that he wants to rise above and be better than the upper levels of the Sith. You apply to a guild that's founded with the express purpose of trying to reform the more decadent impulses of the Empire, and since most of the members are Light-side, accordingly, you figure it's a perfect match.
Yet they reject you because while the two concepts seem compatible, they're really not. The guild wants to reform and improve the Empire, but its members still want to remain a part of the Empire. They're trying to improve the system from within. Your character wants to reject the system and work outside of it. Close cousins, certainly, but just far enough apart that issues can -- and would -- crop up.
You aren't going to get along with the players
This is at once less important and far, far more important because character concepts and agendas can be worked out. Personalities behind the characters are far more dicey.
Some of this does tie into characters. If the rest of the guild is making characters meant to be at least somewhat humorous, your hyper-serious card-carrying member of the Dark Past Association means you're going to be pretty unhappy with the group as a whole. If you create a character looking for love, a guild of stone-faced warriors isn't a good fit. But more important than the reality that different players are looking for different things is the fact that sometimes players are just not going to be compatible.
Case in point: I know of at least one guild that asked for all players to give their first names in real life. At least one applicant refused and was politely declined as a result because the community that the guild was building relied (in part) on knowing the people behind the characters. I didn't have an issue with it myself, since I'm more or less a public figure to begin with, but I can totally understand people who would.
This one stings because it always stings when others tell you that they don't necessarily want to be your friend or let you into the clubhouse or whatever. But it's much better to find this out early on instead of finding it out after several months of arguments and clashes with guild leadership.
You aren't going to be on when the rest of the guild is on
In a lot of situations, this just wouldn't matter. But being on when everyone else is on is kind of a prerequisite to actually roleplaying on a regular basis. Letting you join now would just be a prelude to your ragequit at the lack of people to interact with, and thus it's better to just skip to the ending of the story.
You don't want to do what other players enjoy
I've been a part of roleplaying guilds that have done lots of things. Roleplaying raiding guilds, roleplaying social guilds, roleplaying PvP guilds, roleplaying crafting guilds, even one memorable experience with a roleplaying catering guild. (It didn't last long, but it was a cool idea.) And in every one, I joined because participating in activity X as a part of roleplaying sounded like fun. It was right there out in the open -- this guild plans to do X and roleplay, and if you want to be a part of the guild you should be prepared to do both.
Maybe you're not interested in that. And if not, that's a problem.
I'm not saying that in order to roleplay you should also be excited about doing something else as well. What I'm saying is that not all roleplaying guilds are just general collections of roleplayers. Heck, most games force you to have just the one guild; outside of outliers like Final Fantasy XIV, most games ensure that the people you interact with in your guild are your social framework for all group content. That means you need to be on the same page as your fellow guild members about what that group content entails.
A guild of roleplayers is like any other guild. You might have gotten rejected just because you don't fit in with the rest of the guild, and the fact is that there's nothing wrong with that. Rather than assuming that the other people involved are just jerks, you might be better off just asking about their reasons and figuring out the why first.
Of course, it could also be that they're a bunch of jerks.
As usual, feedback is welcome to firstname.lastname@example.org or in the comments below. Next week is a little up in the air, since one of two things might happen. I may wind up talking about that project I mentioned in the anniversary column... or I might talk about setting up roleplaying projects categorically. We'll find out together!
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.